Air preheater in boiler(pdf): Definition, construction, working principle and types.

This article is about air preheater in the boiler and their function, location, working principle, parts & construction, types, factors affecting while operating air preheater, and their advantages and disadvantages.

Air preheater in the boiler

Air preheater is one of the most important accessories in modern boilers. Air preheater is commonly found in all modern boilers. To increase the efficiency of the boiler accessories like air preheater is used in the boilers.

The function of air preheater in the boiler

The main function of the air preheater is to preheat the air which means the temperature of the input air is increased. Other accessories like economizer and superheater in a boiler do a similar function. This preheating process of air is done before it enters the furnace. The air preheating process is done with the help of hot flue gases.

Location of air preheater in a boiler

Air preheater is an intermediate accessory. It is located in the final stage of the boiler. To increase the efficiency of the boiler, an air preheater is installed between the economizer and chimney. In every modern boiler, an air preheater is commonly installed in the same location.

Air preheater in boiler diagram

air preheaters in boiler diagram
(Air Pre-heater)

This is a simple line diagram of an air preheater in a boiler.

Types of air preheater in a boiler

The air preheaters are classified into two types based on their features and construction. These two types of air preheater are
  • Recuperative or Tubular type air preheater
  • Regenerative or Rotational type air preheater
Recuperative or Tubular type air preheater is further classified into two types, they are
  • Horizontal tubular type
  • Vertical tubular type
Regenerative or Rotational type air preheater is also classified into two types they are
  • Rotating baskets (Ljungstrom)
  • Rotating hood (Rothemuhle)

Tubular type air preheater(Recuperative Type)

Horizontal & vertical tubular type air preheater
(Horizontal & Vertical Tubular Air-preheater)

The above line diagram is a tubular type air preheater. This type of preheater is constructed with a large number of vertical or horizontal tubes. These tubes are used to transfer flue gases. These tubes are fitted along with baffles at certain spaces Which manage the airflow. Soots is collected in the soot hopper at the bottom.

Construction of tubular type air preheater

This type of preheaters is mainly constructed with vertical or horizontal tubes. low carbon steels are mainly used to construct both the air preheaters(AH),  when the AH operates above $ \displaystyle {{450}^{o}}C$.
COR-TEN(wearing steels) are used for constructing AH tubes that operate in the low end or medium temperature. The tubes are expanded by expanders and also welded in rare cases. Welding should be done properly without causing any damage to the tube in AH due to welding heat.
The length of these tubes is about 5 m & 5.5 m in horizontal and vertical tubular air preheater. In horizontal type air heaters, there is no intermediate support for the tubes but in vertical type, there is support at both top and bottom ends of the tubes.
These tubes are about 2.0 to 2.3 mm in thickness. This thickness is for normal fuel usage in boilers. If coal is used as fuel for the boiler, the thickness must be increased due to dust present in the flue gas of the fuel.
The airflow is outwards to the tube in vertical air preheater but in horizontal type AH the air flows in the inline arrangement which is easier for the cleaning process. Soot blowers are not needed in vertical type air preheater.

Working principle

The hot flue gases from the economizer or boiler enter the air preheater through the flue gas inlet at the top. And these gases are allowed to pass through the vertical tubes to the outlet or chimney.
While flue gases pass through the tube, atmospheric air is allowed to pass over the tubes. The atmospheric air enters the preheater through the air inlet.
Both air and flue gases are allowed to flow in opposite directions. The airflow over the tubes is repeated and controlled by baffles. These tubes absorb the heat from flue gases and transfer to the atmospheric air. A soot hopper is fitted at the bottom of the preheater to collect the soot.
In this way, the air is preheated and supplied to a boiler for further combustion.

Rotary Air preheater

In a rotary air preheater, the rotor acts as the main component to transfer the heat. The rotor is installed with several sheet elements such as corrugated sheets, undulated sheets, and flat sheets. The flat sheets are kept separated by a certain distance of about 5 to 10 mm from the other two plates, which helps in creating flow paths and helps in increasing heat supplied and turbulent flow of gas or air.
The thickness of these sheets is about 0.44 to 1.22 mm. The high heat and high turbulence can be achieved by a certain space between the sheets. Like tubular type AH, carbon steel and   COR-TEN is used to manufacture these sheets for both high and low-temperature condition.
To reduce corrosion and deposition of unwanted materials in high operating temperatures, thin sheets are used with minimal spacing distance. Likewise in low or medium operating temperatures, thick sheets are used with maximum spacing distance. Thick sheets always have a long lifespan than thinner sheets.

Rotating baskets (Ljungstrom)

rotary type air preheater
(Ljungstrom and  Rothemühle rotary type air preheater)

Rotating baskets are attached or installed along with the rotor, it has both inlet and outlet to transfer air and gas. These baskets are filled with three types of sheets as mentioned above. The rotor usually rotates at the speed of 4 to 5 rpm when it is driven by an external rotating drive or motor.

The whole setup is covered by a casing which helps in preventing the air from escape. Other seals on the sides also help to prevent leakage of air. Soot blown process always must be done before starting and after finishing the entire preheating process in the air preheater. This helps to keep the AH clean and free from unburnt fuel.
This air preheater is further classified into two types based on the position of the rotating shaft. They are horizontal shaft design and vertical shaft design. Both types are commonly used in utility boilers. While comparing to vertical shaft AH, horizontal shaft types are more popular.

Rotating hood (Rothemuhle)

As the name indicates, only the hood rotates in this type. As we discussed in the last type the rotor never rotates here only air hood rotates. Other components such as stator, baskets filled with heat transfer sheets tend to remain stationary. Both upper and lower air hood rotates and bypasses the air with dampers integrated with the rotating air preheaters.

Factors affecting while operating air preheaters

Some factors affect air preheaters while operating. The common factors are
  • Corrosion at low temperature
  • Fouling and plugging
  • Erosion
  • Fire

Corrosion at low temperature

This problem occurs commonly in tubular type air preheater. Whenever the temperature metal tube falls below the dew point temperature the corrosion occurs. Sulphur content and moisture content present in fuel and flue gas are the main reason for corrosion.
This may cause a serious problem so it requires a regular periodic check-up in the coldest area in the air preheater. The coldest area of air preheater is where the exit gas and cold incoming air meets. Proper maintenance is required to handle this problem.

Fouling and plugging

Fouling means deposition of residual ash content. This usually happens at the low end of the air preheater where the moisture content gets reacts with the moisture at the low-temperature end. This results in fouling. Due to regular deposition, the ash deposit grows bigger in size.
If the size of the ash deposit continuously grows, then the gas passages become plugged. Along with the increase in deposit also results in increasing the pressure drop. Soluble deposits can be removed by water washing and soot blowing must be done at regular intervals to avoid fouling.
Soot blown is impossible in vertical tubular air preheater, so the tube is installed with larger diameters to manage ash deposition.

Erosion

Erosion in air preheater occurs due to high gas velocity. A place or surface in AH, where the gas velocity reaches the maximum point, then it is subjected to erosion. In horizontal tubular AH, a dedicated material called erosion shield is installed.
It has a thickness of about 3mm. So this shield protects the tubes from corrosion. In vertical tubular AH temporary replaceable sleeves are fitted to protect entire tubes from corrosion. In rotary type AH, seals are heavily damaged due to erosion. So to avoid this some dedicated materials are used.

Fire

Fire factors are common in rotary AH. They usually occur due to the unburned coal and oil firing while startup shutdown. By removing the furnace we can avoid fire because it removes all unwanted unburnt gases and fuels. Also by removing gathered ashes and scattered ashes the fire in AH can be prevented. Soot blown must be done at regular intervals to avoid a fire in AH.

Advantages of air preheater in a boiler

  • The efficiency of the boiler is increased.
  • It is used as a source of heat for heating the air.
  • Fuel usage is reduced.
  • Quality combustion is achieved.
  • Furnace losses are reduced.

Disadvantages of air preheater in a boiler

  • Corrosion is the main disadvantage of the air preheater.
  • Erosion in AH due to high gas velocities.

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